Techniques for polynucleotide detection have found widespread use in basic research, diagnostics, and forensics. Polynucleotide detection can be accomplished by a number of methods. Most methods rely on the use of the polymerase chain reaction (PCR) to amplify the amount of target DNA.
The TaqMan™ assay is a homogenous assay for detecting polynucleotides (see U.S. Pat. No. 5,723,591). In this assay, two PCR primers flank a central probe oligonucleotide. The probe oligonucleotide contains a fluorophore and quencher. During the polymerization step of the PCR process, the 5′ nuclease activity of the polymerase cleaves the probe oligonucleotide, causing the fluorophore moiety to become physically separated from the quencher, which increases fluorescence emission. As more PCR product is created, the intensity of emission at the novel wavelength increases. However, background emission can be rather high with this method, due to the required separation of the fluorophore and quencher in the probe oligonucleotide.
Molecular beacons are an alternative to TaqMan for the detection of polynucleotides (see U.S. Pat. Nos. 6,277,607; 6,150,097; and 6,037,130). Molecular beacons are oligonucleotide hairpins which undergo a conformational change upon binding to a perfectly matched template. The conformational change of the oligonucleotide increases the physical distance between a fluorophore moiety and a quencher moiety present on the oligonucleotide. This increase in physical distance causes the effect of the quencher to be diminished, thus increasing the signal derived from the fluorophore.
The adjacent probes method amplifies the target sequence by polymerase chain reaction in the presence of two nucleic acid probes that hybridize to adjacent regions of the target sequence, one of the probes being labeled with an acceptor fluorophore and the other probe labeled with a donor fluorophore of a fluorescence energy transfer pair. Upon hybridization of the two probes with the target sequence, the donor fluorophore interacts with the acceptor fluorophore to generate a detectable signal. The sample is then excited with light at a wavelength absorbed by the donor fluorophore and the fluorescent emission from the fluorescence energy transfer pair is detected for the determination of that target amount. U.S. Pat. No. 6,174,670B1 discloses such methods.
Sunrise primers utilize a hairpin structure similar to molecular beacons, but attached to a target binding sequence which serves as a primer. When the primer's complementary strand is synthesized, the hairpin structure is disrupted, thereby eliminating quenching. These primers detect amplified product and do not require the use of a polymerase with a 5′ exonuclease activity. Sunrise primers are described by Nazarenko et al. (Nucleic Acids Res. 25:2516-21 (1997) and in U.S. Pat. No. 5,866,336.
Scorpion probes combine a primer with an added hairpin structure, similar to Sunrise primers. However, the hairpin structure of Scorpion probes is not opened by synthesis of the complementary strand, but by hybridization of part of the hairpin structure with a portion of the target which is downstream from the portion which hybridizes to the primer.
DzyNA-PCR involves a primer containing the antisense sequence of a DNAzyme, an oligonucleotide capable of cleaving specific RNA phosphodiester bonds. The primer binds to a target sequence and drives an amplification reaction producing an amplicon which contains the active DNAzyme. The active DNAzyme then cleaves a generic reporter substrate in the reaction mixture. The reporter substrate contains a fluorophore-quencher pair, and cleavage of the substrate produces a fluorescence signal which increases with the amplification of the target sequence. Dzy-PCR is described in Todd et al., Clin. Chem. 46:625-30 (2000), and in U.S. Pat. No. 6,140,055.
There is a need in the art for novel probes and methods to detect nucleic acid sequences and monitor amplification reactions, particularly in real time. There is also a need to improve the sensitivity, signal-to-noise ratio, specificity, allelic discrimination, and ease of design of such probes and detection methods.